Pigment epithelium-derived factor, otherwise known as pigment epithelium differentiation-factor, was identified in the conditioned medium of cultured fetal human retinal pigment epithelial cells as an extracellular neurotrophic agent capable of inducing neurite outgrowth in cultured human retinoblastoma cells (Tombran-Tink et al. (1989) Invest. Ophthalmol. Vis. Sci., 30 (8), 1700–1707). The source of PEDF, namely the retinal pigment epithelium (RPE), may be crucial to the normal development and function of the neural retina. A variety of molecules, including growth factors, are synthesized and secreted by RPE cells. Given that the RPE develops prior to and lies adjacent to the neural retina, and that it functions as part of the blood-retina barrier (Fine et al. (1979) The Retina, Ocular Histology: A Text and Atlas, New York, Harper & Row, 61–70), the RPE has been implicated in vascular, inflammatory, degenerative, and dystrophic diseases of the eye (Elner et al. (1990) Am. J. Pathol., 136, 745–750). In addition to growth factors, nutrients and metabolites are also exchanged between the RPE and the retina. For example, the RPE supplies to the retina the well-known growth factors PDGF, FGF, TGF-α, and TGF-β (Campochiaro et al. (1988) Invest. Ophthalmol. Vis. Sci., 29, 305–311; Plouet (1988) Invest. Ophthalmol. Vis. Sci., 29, 106–114; Fassio et al. (1988) Invest. Ophthalmol. Vis. Sci., 29, 242–250; Connor et al. (1988) Invest. Ophthalmol. Vis. Sci., 29, 307–313). It is very likely that these and other unknown factors supplied by the RPE influence the organization, differentiation, and normal functioning of the retina.
In order to study and determine the effects of putative differentiation factors secreted by the RPE, cultured cells have been subjected to retinal extracts and conditioned medium obtained from cultures of human fetal RPE cells. For example, U.S. Pat. No. 4,996,159 (Glaser) discloses a neovascularization inhibitor recovered from RPE cells that is of a molecular weight of about 57,000+/−3,000. Similarly, U.S. Pat. No. 1,700,691 (Stuart), U.S. Pat. No. 4,477,435 (Courtois et al.), and U.S. Pat. No. 4,670,257 (Guedon born Saglier et al.) disclose retinal extracts and the use of these extracts for cellular regeneration and treatment of ocular disease. Furthermore, U.S. Pat. No. 4,770,877 (Jacobson) and U.S. Pat. No. 4,534,967 (Jacobson et al.) describe cell proliferation inhibitors purified from the posterior portion of bovine vitreous humor.
PEDF only recently has been isolated from human RPE as a 50-kDa protein (Tombran-Tink et al. (1989) Invest. Ophthalmol. Vis. Sci., 29, 414; Tombran-Tink et al. (1989) Invest. Ophthalmol. Vis. Sci., 30, 1700–1707; Tombran-Tink et al. (1991) Exp. Eye Res., 53, 411–414). Specifically, PEDF has been demonstrated to induce the differentiation of human Y79 retinoblastoma cells, which are a neoplastic counterpart of normal retinoblasts (Chader (1987) Cell Different., 20, 209–216). The differentiative changes induced by PEDF include the extension of a complex meshwork of neurites, and expression of neuronal markers such as neuron-specific enolase and neurofilament proteins. This is why the synthesis and secretion of PEDF protein by the RPE is believed to influence the development and differentiation of the neural retina. Furthermore, PEDF is only highly expressed in undifferentiated human retinal cells, like Y79 retinoblastoma cells, but is either absent or down-regulated in their differentiated counterparts. Recently, it was reported that PEDF mRNA is expressed in abundance in quiescent human fetal W1 fibroblast cells and not expressed in their senescent counterparts (Pignolo et al., 1993).
Further study of PEDF and examination of its potential therapeutic use in the treatment of inflammatory, vascular, degenerative, and dystrophic diseases of the retina and central nervous system (CNS) necessitates the obtention of large quantities of PEDF. Unfortunately, the low abundance of PEDF in fetal human eye and furthermore, the rare availability of its source tissue, especially in light of restrictions on the use of fetal tissue in research and therapeutic applications, make further study of PEDF difficult at best. Therefore, there remains a need for large quantities of PEDF and equivalent proteins. Accordingly, the obtention of nucleic acids that encode PEDF and equivalent proteins, and the capacity to produce PEDF and equivalent proteins in large quantities would significantly impact upon the further study of PEDF, its structure, biochemical activity and cellular function, as well as the discovery and design of therapeutic uses for PEDF.